Conventional broadcast NTSC composite color subcarrier television signals nominally occupy assigned broadcast channels which are 6 MHz wide. The visual carrier frequency (amplitude modulation or "AM") is nominally 1.25 MHz above the lower boundary of the channel. The aural or sound carrier frequency (frequency modulation or "FM") is centered 4.5 MHz above the visual carrier frequency; and the chrominance subcarrier is 3.579545 MHz above the visual carrier frequency.
As is well known, since the color subcarrier frequency is an odd multiple of one half of the line scan rate, the sideband signals carrying the chrominance information exist as energy groups which are interleaved within the high frequency energy groups of luminance. When the chrominance and luminance energy groups overlap spectrally, the resultant picture display may be characterized by cross-color artifacts and cross luminance artifacts. Such artifacts may be noticeably reduced by comb filter encode and decode processing such as is taught in the present inventor's U.S. Pat. No. 4,731,660, the disclosure of which is hereby incorporated by reference.
Recently, with the advent of improvements in television picture display systems, workers in the art have proposed a wide variety of improved resolution television systems, one of which is known as "high definition television" or "HDTV", which proposes new transmission standards, typically 1125 scan lines, which are not immediately compatible with existing television receiving devices. Scan line doubling techniques, such as described in U.S. Pat. No. 4,719,644 are also known in the prior art as a way to increase virtual resolution.
One proposal for "compatible" high definition television is set forth in an article co-authored by Rzeszewski, Pazarci and LoCicero entitled "Compatible High-Definition Television Broadcast Systems" IEEE Trans. on Broadcasting, Dec. 1987 issue. That article sets forth systems using two standard, adjacent 6 MHz channels and which were capable of increased horizontal and vertical resolution, a wider aspect ratio and reduced cross-talk between the various components of the composite signal. In their proposal the luminance signal is given the benefit of over 7 MHz of bandwidth, which results in a much sharper picture image display.
The present inventor has discovered that increased resolution provided by two-channel television systems solves but one aspect of a multi-faceted problem. Most of the facets of this problem do not relate to present signal format limitations on resolution. Rather, most relate to unwanted noise and interference artifacts such as short term ringing and echo due to vestigial sideband filter group delay errors, and ghosts from multipath transmission phenomena.
More particularly, in cable television systems, the apparent defects of the NTSC format, such as cross-color, cross-luminance artifacts and lack of resolution are of less concern than the more pressing problems of adjacent channel interference, noise and short term ghosts and ringing (e.g. of 300-500 nanosecond duration) due to deficient line terminations associated with distribution amplifiers in local distribution loops. Therefore, the present inventor's pending application, Ser. No. 07/151,895, filed on Feb. 3, 1988 and entitled: "Wideband NTSC Compatible Transmission System with Noise Reduction Processing" proposes the use of a transmission system which uses two adjacent channels, not only to increase picture resolution but also to reduce noise and interference artifacts.
Thus, it is presently known to employ a 12 MHz spectrum for carrying a bandwidth increased, NTSC compatible, quadrature modulated color subcarrier signal which exhibits a better signal-to-noise ratio and protection against ghosts and interferences from other channels while obtaining resolutions approaching those obtainable with 35 millimeter format motion picture films. While the inventor's referenced wideband system is very useful for applications where bandwidth is not the issue (such as closed-circuit theatrical displays, or pay cable television services willing to offer the highest in technical picture quality), there are many situations where the use of spectrum allotted to two channels for transmission of but a single television program is not practical.
Over-the-air transmission is clearly limited to the number of available channels within the alloted radio frequency spectrum Even most present cable systems have a reasonably small number of channels available. Further, bandwidths of recording devices (video tape recorders, video discs) are limted by the characteristics and bandwidth restraints of the recording media.
Workers in the art have proposed NTSC compatible, single channel systems providing increased quality television picture signals. One such proposal is reported in an article co-authored by T. Fukinuki and Y. Hirano entitled "Extended Definition TV Fully Compatible with Existing Standards", IEEE Trans. on Communications, Vol. COM-32, No. 8, PP. 948-953, (Aug. 1984). Another proposal is reported in an article coauthored by Y. Yasumoto, S. Kageyama, S. Inouye, H. Uwebata and Y. Abe entitled "An Extended Definition Television System Using Quadrature Modulation of the Video Carrier with Inverse Nyquist Filter", IEEE Trans. on Consumer Electronics, Vol. CE-33, No. 3, pp. 173-180 (Aug. 1987). One more proposal is the subject of a paper presented by Dr. Michael Isnardi entitled "Exploring and Exploiting Subchannels in the NTSC Spectrum" at the 129th SMPTI Technical Conference, Oct. 31-Nov. 4, 1987, at Los Angeles, California. All of these proposals have related to multiplexing higher definition or wider picture aspect ratio information into the existing 6 MHz spectrum; and, they do not address the specific problems of transmission noise and unwanted artifacts which are very significant contributors to picture degradation.
The present inventor in his U.S. Pat. No. 4,262,304 described a low/level signal booster for television transitions which added an adaptively pre-emphasized noise reduction boost signal to a main path signal before the signal was subject to degradation by virtue of passage through a bandwidth degrading medium, such as a video recorder or noisy transmission path. Another prolific worker in the field, Ray Milton Dolby proposed in his U.S. Pat. No. 3,846,719 that a pre-emphasis/noise reduction signal be carried on a separate path or recording track parallel with the main video path or track.
Yet, despite these prior efforts a hitherto unsolved need has existed for a single channel (6 MHz bandwidth) television transmission system which contains pre-emphasized noise reduction information for reducing noise, interference and short term ringing and yet which remains fully compatible with the present NTSC signal format, so that television receivers of the prior art which are not equipped to process the noise reduction information are not deprived of their conventional functionality.